Patent document EP 1460890 discloses the technique of bonding layers of a multilayer printed circuit board to one another by means of induction. To that end, the sheets making up the layers containing a circuit image are provided with a perimetral reserve area without a circuit image but provided with several turns in short-circuit or metal spots which are superimposed when these layers containing a circuit image are stacked with resin layer interposition to form a bundle. The bonding method comprises the step of generating a magnetic field traversing the bundle precisely through said turns or spots, electric currents being induced therein which due to the Joule effect generate enough heat to melt the resin layers in the adjacent areas, which layers set upon polymerization and cause the irreversible bond between the layers containing a circuit image.
In the scope of the invention, multilayer printed circuit boards are increasingly complex in line with the growing requirement for quality and compactness and this generates new needs that did not exist until now. Such is the case of multilayer printed circuit boards having a high internal layer interconnection density, and especially those combining rigid circuit areas with flexible areas which require using sheets of the order of microns thick and materials which do not have a predictable mechanical expansion or shrinkage behavior. This causes misalignments to occur between the rigid internal layers and the flexible layers once the bundle is formed during its handling in subsequent manufacturing operations, causing a high level of rejection or of defective material.
To solve these drawbacks, welding anchors or bonds are considered for use between the layers of the circuit at non-peripheral points of the sheets, i.e., outside the reserve area mentioned above. In short, it is desirable to be able to provide the layers with suitable inner areas to enable induction welding said layers in order to anchor all the internal rigid and flexible layers of a circuit for controlling or for making the expansion and/or shrinkage movement of said layers more predictable and consequently reducing the misalignment effect.
On the other hand, to perform the induction welding an upper magnetic conductor is conventionally arranged on the upper face of the formed bundle coinciding with a turn or metal spot of the upper sheet and a lower magnetic conductor is applied on the lower face of the bundle coinciding with the corresponding turn or metal spot of the lower sheet, and a magnetic field confined by said conductors traversing the bundle is generated. To perform this operation, said bundle must be set in place or supported by a tool which prevents side movements both of the bundle and of its internal layers. At the same time, this tool must be prevented from disturbing the magnetic field traversing the bundle or from being heated due to the effect of the magnetic field and become an unwanted heat source. For such purpose, the known tools formed by a single or several assembled parts have a plate shape that does not completely cover the lower face of the bundle rather said plate is provided on its periphery with recesses or openings through which the lower magnetic conductor accesses the lower face of the sheet making up the lower layer of the bundle. Said recesses or openings revealing the bundle from the underside coincide with the reserve area provided in the sheets containing a circuit image of the bundle, generally having standard dimensions such that one and the same tool can be used for manufacturing different bundles or multilayer printed circuit boards.
In contrast, if bonds are to be made between layers through internal points of the sheets making up same, custom-made tools must be made for each circuit since the turns or metal spots are housed in the sheets depending on the particular circuit images for each printed circuit board to be manufactured. In fact, the designer first designs the circuit and then selects the location of the turns or metal spots according to the space available between the circuit images in the sheets. In the event of using conventional tools, these must be provided with openings precisely in those areas in which the turns or metal spots will be located to allow applying the lower magnetic conductor on the lower face of the sheet making up the lower layer of the bundle coinciding with the mentioned turns or metal spots.
A first objective of the present invention is a versatile tool which can be used for manufacturing different multilayer printed circuit boards, which therefore serves for bonding layers through internal points thereof even when the turns or metal spots are located in each circuit or bundle at different points with respect to one and the same reference axis.
The tool must further assure high reliability, in the sense that it must offer an absolutely flat support surface for the bundle so that said bundle does not warp and its layers are completely flat during the bonding operation.
The same tool must further be suitable so as to not disturb the magnetic field confined by the magnetic conductors and so as to not heat up due to the effect of the magnetic field.
Patent document WO 2008028005 describes a machine suitable for bonding the layers of a bundle of a multilayer printed circuit board allegedly advantageous with respect to the technology described in document EP 1460890. While document EP 1460890 describes the use of welding electrode pairs assembled facing one another in respective arms of a C-shaped magnetic core and having the capacity to move towards or away from one another for applying each one on opposite faces of the bundle and confining a magnetic field traversing the bundle from one electrode to another, patent document WO 2008028005 describes the use of an E-shaped magnetic core intended to be applied on a face of the bundle for confining a magnetic field the field lines of which traverse the bundle through three areas from each of the outer legs of the E to the central leg when these are applied on a face of the bundle. According to WO 2008028005, while the bond between the arms of the C prevents the electrodes from being able to be arranged in the inner areas of the bundle by abutting with the edge of the bundle, the solution in E allows arranging the magnetic core freely on any area of the bundle, therefore allowing unlimited mechanical bonds between the layers of the bundle through any internal point. However, document WO 2008028005 does not indicate or suggest what may be the intention or what the circumstances are that make performing this bonding in inner areas be of interest.
Another objective of the present invention is a method for performing the operation of bonding the layers of a bundle of a multilayer printed circuit board which allows performing said bonding at internal points of the panels making up the mentioned layers, outside the reserve area thereof in an easy and safe manner for the circuit.
The lack of a machine especially suitable for carrying out a method encompassing the objectives defined above is also stressed.